91
CHAPTER THREE
Cells
Nuclear
pores
Nucleus
Nucleolus
Chromatin
Nuclear
envelope
(a)
(b)
FIGURE 3.19
The nucleus is the genetic headquarters of the cell.
(
a
) The nuclear envelope is selectively permeable and allows certain
substances to pass between the nucleus and the cytoplasm. Nuclear
pores are more complex than depicted here. (
b
) Transmission electron
micrograph of a cell nucleus (7,500×). It contains a nucleolus and
masses of chromatin.
TABLE
3.2
|
Structures and Functions of Cell Parts
Cell Parts
Structure
Function
Cell membrane
Membrane mainly composed of protein and lipid
molecules
Maintains integrity of the cell, controls the passage of materials
into and out of the cell, and provides for signal transduction
Ribosomes
Particles composed of protein and RNA molecules
Synthesize proteins
Endoplasmic reticulum
Complex of connected, membrane-bound sacs, canals,
and vesicles
Transports materials in the cell, provides attachment for
ribosomes, and synthesizes lipids
Vesicles
Membranous sacs
Contain substances that recently entered the cell, store and
transport newly synthesized molecules
Golgi apparatus
Group of F
attened, membranous sacs
Packages and modi±
es protein molecules for transport and
secretion
Mitochondria
Membranous sacs with inner partitions
Release energy from food molecules and convert the energy
into a usable form
Lysosomes
Membranous sacs
Contain enzymes capable of digesting worn cellular parts or
substances that enter cells
Peroxisomes
Membranous vesicles
Contain enzymes called peroxidases, important in the
breakdown of many organic molecules
Centrosome
Nonmembranous structure composed of two rodlike
centrioles
Helps distribute chromosomes to new cells during cell division,
initiates formation of cilia
Cilia
Motile projections attached to basal bodies beneath the
cell membrane
Propel F
uids over cellular surface
²lagella
Motile projections attached to basal bodies beneath the
cell membrane
Enable sperm cells to move
Micro±
laments and microtubules
Thin rods and tubules
Support cytoplasm, help move substances and organelles
within the cytoplasm
Nuclear envelope
Porous double membrane that separates the nuclear
contents from the cytoplasm
Maintains the integrity of the nucleus, controls the passage of
materials between the nucleus and cytoplasm
Nucleolus
Dense, nonmembranous body composed of protein and
RNA molecules
Site of ribosome formation
Chromatin
²ibers composed of protein and DNA molecules
Carries information for synthesizing proteins
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